Ceramic materials, which have many characteristics such as chemical stability, corrosion resistance, and usability in high-temperature environments in comparison to metals, have high hardness or strength but have the drawback of low toughness, and the application thereof is limited. Further, the enhancement of reliability is viewed as important when a ceramic is used as a structural body. In order to enhance the reliability of a ceramic structure, the structure must be a dense body. To obtain a dense body, it is necessary to reduce voids—that is, the air holes in the sintered body, by sufficiently increasing the sintering temperature. However, when the sintering temperature is increased, the grains of the sintered compact become large, which causes chipping such as grain pulling out from the sintered compact surface. This becomes a source of fracture, which leads to drawback such as reduction in the structural strength of the ceramic. Further, in the case of a ceramic structure containing a plurality of metal oxides, unless the growth of sintered compact crystals of each metal oxide that is contained is controlled harmoniously, problems arise such as dominance of the characteristics of certain types of metal compounds, leading to the lack of structural functionality as a ceramic body which should be inherent to the composition.
In this way, achieving both the densification of a ceramic structure and the size reduction of sintered compact crystals has been a problem in the preparation of ceramics with guaranteed reliability.
As one method of enhancing the reliability of a ceramic structure and enhancing the toughness—that is, the fracture toughness value (KIC)—of a ceramic, it has been disclosed in Patent Document 1, for example, that a ceramic with high strength/toughness, which is a dense sintered compact having small crystals and simultaneously exhibiting a bending strength (σb) of not less than approximately 1,000 MPa (1 GPa) and a KIC of not less than 15 MPa·m1/2, is obtained by preparing a ZrO2 (0.3 to 1.7 mol % Y2O3)-25 mol % Al2O3 solid solution powder containing zirconia (ZrO2) to which yttrium oxide (Y2O3) having a function of high toughness is added, and alumina (Al2O3) having a function of high strength, by means of a sol-gel method and performing Pulsed Electric-Current Pressure Sintering (PECPS). However, in this case, there remains a problem in that the raw material preparation cost is high (the cost per 1 g of raw material is over several thousand yen).
On the other hand, in the case of a powder prepared by a solid phase reaction or the like or a powder prepared by a liquid phase method other than a sol-gel method, the composition or particle size distribution of the powder particles may not be uniform. Therefore, even when these powders are sintered, the composition in the ceramic structure may not be uniform, and although the powders are sintered at a high temperature in order to increase homogeneity, the aforementioned problems of densification and size reduction of the sintered compact crystals may not be satisfactorily resolved. Thus, the availability of a ceramic with high strength/toughness is still a problem.
Therefore, in order to resolve the problems described above, the applicants of the present invention and others proposed, in Patent Document 2 below, the production of an oxide-based ceramic (ZrO2—Al2O3-based ceramic) which simultaneously exhibits high strength and toughness by preparing a zirconia-alumina-based microparticulate powder containing a small amount of yttrium oxide (1.5 mol % Y2O3) by a coprecipitation process and then molding the powder at a high density and sintering the sintered compact crystals so as to be dense and minute by Pulsed Electric-Current Pressure Sintering (PECPS) at a high heating rate.
However, PECPS has restrictions with regard to the shape of the compact due to uniaxial pressure sintering, and the productivity is also low. From the perspective of cost reduction as well, there are many problems regarding developing ceramics to a wide range of applications.